Decoupling unit



1m30, 194,5. J E; @SfER 2,368,474Y

DECOUPLING UNIT FiledSept. 24, 1942 l Inventor:

' James E. Keist'en' by W His torney .Patented 1m30, 194s nEcoUrLmG UNrr James E. Kemer, scena, N. Y., assignmtoceneral Electric Company, a corporation of New York Application september 24, 1942, serial No. 459495 v 4 claims. (ci. 179-111) My invention relates to filter units, in particular to filter units for interlstage decoupling of power leads in high frequency amplifiers, and

has for. its object to provide a lter which is non-radiating and which has large attenuation at high frequencies, but practically no attenuation at power frequencies.

In high gain ampliers without frequency conversion, especially when employed at very high frequencies, the problem -of multi-stage feed-back is quite serious. Even when complete shieldingl of each ,stage is included, the power leads supplying voltages to the electron vdischarge devices in the different stages cause feed-back between succeeding stages. It is an object of my invention to provide an improved decoupling unit for the power lead of a' given stage at its point of entry into a higher level stage which reduces feed-back between the stages and which may be used in cascade for` several or more stages.

A further object is to provide a decoupling unit which may be easily 'constructed from small standard filter elements and which is not critical to frequency.

The features of my invention which I believe to be novel are setA forth with particularity in' the appended claims. My invention itself, however, together withfurther objects'and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying ldrawing, in which Fig. 1 shows a preferred form of the decoupling unit of my invention; Fig. 2 shows how the unit of Fig.v 1 may be used -in cascade decoupling of successive stages of a radio frequency amplifier; Fig. 3 illustrates the manner in which a plurality of the filter circuits of Fig. 1 are used in eachl stage to' obtain a maximum deycoupling between the stages;J and Figs. .4 and 5` show modifcations'o'f the filter. of Fig. l:

Referring to Fig. 1, the shown as' inserted in series with the power leads I I and I2 which may supply vltage respectively to a given stage and a higher level stage of a "radio frequency amplifier (not shown), each of/ said stages being completely shielded, in wellkn'own manner, by a metallic shield I3. The decoupling unit I0 comprises theflxed resistor, I4,v

preferably of the insulated type familiar to those the power leads II andV I2a t the points I5 and I6.A Connected in shunt 'to the 'resistor I4,' be,

decoupling unit vIll is tween the points |5- and I6, and spirally ar'- 'ranged on the outer surface of resistor I4 is a conductor I1 having a resistance of relatively low value. The individual turns ofv spirallyI arranged conductor I1 are closely spaced so that the conductor I1 acts as an lnductance coil and presents a path in shunt to resistor I4 having a low resistancefor, the supply current .and a high reactance for radio frequency current. Surrounding the spirally arranged conductor Il is a layer I8 of suitable insulating material, such as varnishedtape or cambric. Preferably, the conductor Il is wound tightly about the vthe ret sistor I4 and the layer I8-of insulating` material, is Wound tightly about the conductor l1. A tube or casing I9, of any suitable conductive material, such as copper, encloses theJ decoupling unit thus far described and forms therewithl a small unitary structure. The tube or casing I9 is secured to the shield I3 in any desirable manner, vsuch as by soldering or brazing. A capacitor 20 is also provided for bye-passing high-frequency .currents from the power lead II and is connected by a very short lead" 2| to conductor to the lower level stageof the two amplifying v stages between -which the decoupling unit I0 is to be inserted. Radio frequency currents, picked up by the conductor ,l2 in the higher level stage,

are filtered out by means of ythe high reactance .of conductor lIl in series with the low reactance of capacitor 20. The spirally arranged conductor I1 of the unit acts both as a path of low re- I sistance for power currents supplied over leads Hand I2 and as a radio frequencychoke to suppress any high .frequency energy picked up by the lead I2 before'leaving the shielded com,- partmenty of the higher level stage of the amplifier. By proper choice vof the size of conductor Il, a negligible power voltage drop occurs in the lter unit I0. The casing I9 serves tof/shield thev inductance coil I1 to prevent any feed-back L ofradio frequency energy by this coil.r Since '.60 skilled in the radio art and having a relatively r high` resistancefor example; 5000 ohms. The ends of resistor- Il are connected` respectively to to prevent ex there is a tendency, at high frequencies,"for the I inductance I1l to, resonatewith the capacitor 2li, the resistor! Il is provided as a damping means lessive resonances in the coil of conductor I1 or in the inductance and capacitor combination. Preferably, the leads 2l and 22 `conilliing capacitor 20 respectively. to the Ipower lead II and the casing so that the inductance low and, hence, a very low reactance can be achieved between point I5 and ground.

In Fig. 2 is shown schematically a cascade arrangement of the decoupling unit I for a plurality of stages 3I-35 of the radio frequency amplifier, the level of the radio frequencyrenergy progressing successively from stage 3| to stage 35. As shown, each of the stages is completely shielded, the shields I3 being grounded. A suitable source of power, such as the battery 36, is directly connected by means of conductor I2 to the decoupling unit III in the highest level stage 35, power for thelower level stages being supplied by conductor II.

The cascade decoupling, as shown in Fig. 2, has the advantage over other types of decoupling in that, as the power circuit progresses to a more and more sensitive stage, greater and greater feed-back attentuation is introduced. Thus, between the high level stage 35 and the low level stage 3|, there are four decoupling units I Il to prevent feed-back of energy from stage 35 to stage 3|.

Fig. 3 shows how a I9 are made very short of these leads is very single stage of the amplifier, for example the stage 34, may include a plurality of decoupling units, the exact construction of a particular unit depending upon the portion of the circuit to which the power is being supplied. Thus, in Fig. 3 two decoupling units I 0 of the type shown in Fig. l are connected in series with the leads 4| and 42 supplying power respectively for the anode and filament of an electron discharge device 43, the conductor II being the power lead connected to the stage of higher level and conductor II' supplying power to the stage of lower level. The conductor 4I -is connected to the anode of device 43 through a tuned circuit 44. Tuned circuit 45 is inductively coupled to`tuned circuit 44 and supplies high frequency signal potentials to the control electrode of the electron discharge device 46 in the succeeding stage of the amplifier. Bias potential for the control electrode of device 46 is supplied by means of the conductor 41 from any suitable source, not shown, conductor 41' similarly supplying bias potential to the lower level stages of the amplifiers. Since bias potential for other stages of the amplifier is obtained preferably from the same source, in order to prevent feed-back ofy radio frequency energy between stages, the decoupling unit 48 is con# nected in series with the conductor 41 and the tuned circuit 45. This decoupling unit 48 is similar to unit I0 with the exception that the inductance I1 has been omitted. The omission of this inductance is permissible in this instance because either very low or zero power currents are involved in the biasing circuit of the control electrode of device 46. However, since the conductor 41 itself comprises an inductive coupling between the stages sufficient to feed back high frequency energy pickedup in the higher level stage, the use of the decoupling unit is desirable to maintain this feed-back at a minimum.

The decoupling unit 49 connected. in series with the leads A50, 50' supplying potential to the screen electrode of device 43 and of the devices: of the other stages is similar to the unit 48 in that the inductance coil I1 is omitted from the unit. The values of the fixed resistors used in units 48 and 49 depend on the currents flowing in the particular control circuits. ance I of unit 48 may have a higher value of Thus, vresistwatt, resistor as the resistor resistance than the resistor 52 of unit 49, since the grid current of the device 46 is either zero or a very low value whereas the screen grid current is of a greater magnitude. It will be appreciated that in the decoupling units 48 and 49, the metal casing 53 acts as a shield to prevent the pick-up of high frequency energy from the intense field present in the stage.

In Fig. 4 there is shown an alternative method of installing the lter unit. The unit I9 isy located in the higher level stage, being supported on the shield Iii-separating adjacent stages. The capacitor 20 -is located in the lower level stage and is connected by the short lead 2| to the power lead II and bythe short connector 22 directly to grounded inter-stage shield I3. In all other respects the filter unit is the same as the unit shown in Fig. 1.

In the modification shown in Fig. 5 the filter unit is partially located in both of the adjacent stages, extending through an opening in the inter-stage shield I3. The unit may be supported in this position by soldering the casing I9 to the shield I3. The unit I0 in all other respects is the same as the unit of Fig. l.

It is thus seen that my invention provides a filter unit particularly 'adapted for high gain amplifiers operating at high frequencies. By

complete shielding of the filter unit, the pick-up' Yof high frequency energy by the filter elements themselves is prevented. By proper selection of the units, a negligible voltage drop is obtained in the power source, as well as effective suppression of radio frequency energy from the lead connected to the power source. The presence of the fixed resistor in the circuit provides damping which prevents excessive resonances in the inductance coil itself or in the combination of the inductance coil and the by-pass capacitor. Moreover, this decoupling unit is particularly effective when used in cascade in successive stages of the radio frequency amplifier in the manner and for the reasons pointed out above.

In an amplifier operating at 50 megacycles, it was found that by the use of a 500G-ohm, one

I4, 50 turns of 30- mil wire as the spirally arranged conductor I1, and a .005 microiarad condenser as the capacitor 2B, asingle decoupling unit resulted in an attenuation of 40 decibels.

While I have shown a unit illustrated in Fig. l, it will be apparent that various modifications may be made.. Thus, in some installations it may be found desirable to omit the casing I 9. In such a case, the capacitor 20 is directly connected to the stage shield I3. Thus, while I have shown particular embodiments particular decoupling of my invention, it will of course lie understood that I do not wish to be limited thereto since various modifications Inay be made, and I contemplate !by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as `new andl desire to secure by Letters Patent of the United States is:

1. In radio frequency apparatus having a low level stage, a higher level stage, and means common to said stages for supplying power currents thereto, means for filtering high frequency currents from said supplying means comprising, separate shielding means for said' stages, an insulated resistor connected in said supplying means,

, asesma.

` surrounding said conductor and insulated therefrom, said shield being connected t said shielding means, and a capacitor connected between said shield and the end of said resistor connected to said lw level stage.

2. A device for decoupling the power leads be- -tween successive stages of high frequency ampliers comprising, shielding means for each of said stages, an insulated resistor connecteduin series with said le'ad and having a conductor of low resistance value spirally arranged around its outer surface, saidv conductor being connected to both ends of said resistor and forming a. path in shunt thereto having a low resistance and a high reactance, a metal tube insulatingly surrounding said spirally arranged conductor and conductively connected to said shielding means, and a. capacitor connected Ibetween said tube and the end of said resistor connected tothe stage of lower level.

3; A filter unit comprising an insulated resistor shielding means, an inductance coil positioned' 4within said tube and connected in parallel with 1 of large resistance value having spirally arranged around its outer surface a conductor of low resistance value, said conductor being connected to both ends of said resistor and forming a path in shunt thereto having a low resistance and a high reactance, a continuous metal grounding shield surrounding said spirallyarranged conductor and insulated therefrom, and a capacitor connected .between one of said ends and said shield.

4. A high frequency suppressing device for a lead supplying unidirectional vpotential Ato an amplifying stage of radiov apparatus having means,

for shielding said stage, comprising an insulated resistor connectedin series with said lead, a continuous metal grounding tube surrounding said resistor and conductively connected to, said said resistor, and a capacitor connected between said tube and the end of said resistor. 

